Switch to use stestr completely

This commit adds .stestr to .gitignore and remove os-testr from
test-requirements.txt to switch to use stestr completely. We actually
already switched in the previous commit[1]. And now, we can switch
completely.

One thing - .testrepository is still remaining in .gitignore. This
directory can be still remaining in the most of user environments. So
that, to avoid the confusion, '.testrepository' should be still there
for a moment.

[1] I39f5db8f77de027c18917215c761dca69d8cbea0

Change-Id: I9530af1a91ae20dca24a699fbf04893102570f09

.gitignore

@@ -11,6 +11,7 @@ ChangeLog
 *.db
 dist
 build
+.stestr
 .testrepository
 .coverage*
 !.coveragerc

test-requirements.txt

@@ -9,7 +9,6 @@ testtools>=0.9.34
 oslosphinx
 PyMySql
 psycopg2
-os-testr
 oslo.concurrency>=3.5.0
 # releasenotes
 reno>=0.1.1 # Apache2